Free piston water hammer pile driving

ABSTRACT

Driving long piles into submerged lands with a liquid ram or spear generated in a free-piston evacuated tube. Various drivers are enclosed. In one embodiment, the pile itself is used as at least a portion of the working chamber for generating water hammer. In another, the working chamber is a tube separate from the pile.

United States Patent 1 [111- 3,820,346 Wisotsky 1 June 28, 1974 FREE PISTON WATER HAMMER PlLE 3,638,738 2/1972 Varnell 61/535 x DRIVING 3,646,598 2/1972 Chelminski 61/535 Inventor: Serge S. Wisotsky, Sharon, Mass.

Assignee: Orb, Inc., Marion, Ohio Filed: June 30, 1972 Appl. No.: 267,740

Related US. Application Data Continuation-impart of Ser. No. 163,422, July 16, I971, abandoned.

US. Cl 61/535, 173/1, 181/.5 Int. Cl E02d 7/28 Field of Search 6l/53.5, 53.54; 181/63,

References Cited UNITED STATES PATENTS 9/1971- Chelminski 173/1 Primary Examiner-Jacob Shapiro Attorney, Agent, or FirmRobert R. Priddy [5 7] ABSTRACT Driving long piles into submerged lands with a liquid ram or spear generated in a free-piston evacuated tube. Various drivers are enclosed. In one embodiment, the pile itself is used as at least a portion of the working chamber for generating water hammer. In another, the working chamber is a tube separate from the pile. I

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1 FREE PISTON WATER HAMMER PILE DRIVING CROSS-REFERENCE This is a continuation-in-part of prior copending application Ser. No. 163,422, filed July 16, 1971 and now abandoned, the disclosure of which is hereby incorporated by reference.

BACKGROUND The kinetic energy output of a pile driver is a function of its driving mass and its velocity at the instant of impact with a pile. The emplacement of piles in the ground by pile driving is accomplished by transmitting the kinetic energy of a hammer or other driving mass to a pile in sufficient quantity to cover nonproductive energy consuming factors such as impact stresses, radiation, reflection and ground quake, and to overcome the friction, elasticity and inertial impedance compothe environment and loss of invested capital. Thus, to

provide adequate load-bearing and to prevent pull-out, requirements exist for driving piles hundreds of feet long, several feet in diameter, weighing hundreds of tons, and for continuing the driving to depths of soil penetration where driving resistance is severe.

A complex series of relationships pertaining to pile and soil characteristics, driving environment, economics and materials governs the design of a pile driver. However, generally speaking, the advent of piles of greater mass and conditions productive of more severe driving resistance require drivers of increasing kinetic energy output. In the absence of adequate driving energy, that which is available is consumed largely or completely by the aforementioned nonproductive energy consuming factors, leaving little or no energy to drive the pile. Under such conditions, some help is obtained by palliatives such as drilling a pilot hole, water jetting or grouting into an over-size hole, but these measures normally reduce load-bearing capacity. Thus, as each new generation of more massive piles and more severe driving conditions arises, drivers of greater energy output must be designed.

The kinetic energy output of an existing hammer can be increased by increasing either its mass or its impact velocity. The latter alternative is unattractive for a number of reasons.

First, there is the matter of the efiiciency with which the hammer transfers energy to the pile. In a complete inelastic collision between a hammer and pile, the kinetic energy remaining after impact for overcoming the nonproductive factors and driving the pile is in proportion to the ratio of the hammer mass divided by the total mass of hammer plus pile. An increase in pile mass without a corresponding increase in hammer means results in a reduction of driving efficiency.

Also, higher hammer velocities are more predisposed to produce high local impact stresses. When the latter exceed the yield point of the pile material, kinetic energy is wasted and efficiency reduced.

For these and other reasons, manufacturers discourage the use of pile driver in which the hammers 'mass is less than one-fourth that of the pile, and a mass ratio of one-half is generally recommended for land-based operations.

This presents a dilemma in off-shore pile driving. The largest stean hammer pile drivers currently in use in off-shore/marine work are limited, practically, by safety considerations relative to their handling. in stormy weather, to weights on the order of 60 tons (hammer mass about 30 tons). Consequently, they usually are inadequate to drive the larger piles due to massmismatch.

For instance, with a ZOO-ton pile, the energy transfer efficiency of a 30 ton-hammer would be 100 percent X 30/(30 200) or about 13 percent. Moreover, even this relatively small amount of energy transferred to the pile is not altogether effective in driving for other reasons stated below. i

The picture is further complicated by the fact that the energy in a pile is effective to penetrate the soil only if there is a proper impedance match between the force-time-displacement characteristics of the driver and corresponding parametric thresholds of the soil. The available alternatives for varying the force-timedisplacement characteristics of a. steam hammer are limited, and this presents practical problems as the tip and sides of a pile often pass through strata of widely varying characteristics as the pile penetrates the earth.

Thus, under the severest conditions, pile driving is an arduous, time consuming and expensive task which sometimes ends in failure to achieve design loadbearing capacity or depth. Also, the inability to drive large piles to sufficient depths often necessitates driving a larger number of smaller piles, so that as many as eight or sixteen piles may be required for the foundation of a single leg of a multi-leg offshore structure.

Bearing in mind the storm-weather safety considerations mentioned above, it is of interest that at least one pile driver manufacturer has proposed for offshore operations a pile driver, nominally. rated at almost 500,000 ft. lb., weighing on the order of 230 tons, equivalent to the weight of several locomotives. Lifting this gigantic mass and adequately securing it during storm conditions present major challenges. Nevertheless, the fact that at least some of those active in the art seem ready to accept these formidable challenges suggests the severity of the problems and limitations with which the pile driving art is now struggling.

SUMMARY OF THE INVENTION The method of the present invention is carried out in (including on) a pile which has its tip embedded in the ground, including for instance the subsoil of a body of water. An evacuatable enclosure with sidewalls and a lower barrier is effectively coupled with the tip of the pile for transmitting driving forces exerted upon said barrier to said tip. The method comprises: at least partially filling said enclosure with water, providing a hydrostatic head in said water bearing upon said barrier;

evacuating said chamber by pushing on said water with piston means for displacing water along the longitudinal axis of the chamber away from the barrier; releasing said piston and accelerating a mass of water following said piston along the axis of the chamber towards said barrier under the influence of the hydrostatic head, the water and piston moving substantially independent of said pile; suddenly decelerating said piston and mass of water against said barrier, thereby converting hydraulic kinetic energy to a water hammer driving pulse for driving said pile into the ground; and repetitively evacuating, accelerating, decelerating and driving as aforesaid. Using this method, it is possible to generate powerful mechanical impules whose pressure-time characteristics can be tailored over a wide range of values to match corresponding requirements of the pile and soil conditions. Although the invention was developed for and provides special benefits for underwater pile driving, it also has applications in the driving of piles on land. Various other advantages will be discussed along with certain preferred embodiments of the invention illustrated in the accompanying drawings and text.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic diagram of a free piston evacuatable enclosure water hammer in which the piston is reset by mechanical means.

FIG. 2 is a sectional view of a free piston type evacuated tube water hammer driver module with condensable vapor reset.

FIG. 3 is a partly sectional, partly schematic view of a double free piston driver capable of driving in either direction; FIG. 3A is a schematic diagram of an alternative form of a portion of FIG. 3.

FIG. 4 is a schematic diagram of a free piston evacuatable enclosure water hammer with condensable vapor reset and a water reservoir which is capable of nonsubmerged operation.

DESCRIPTION OF PREFERRED EMBODIMENTS FIG. 1 discloses one example of that class of evacuatable tube water hammer drivers which include one or more free pistons. In the present context, a free piston is one which, during at least a portion of its movement between the extreme limits of its travel, is not directly coupled to or is able to move at least substantially independent of, the pile.

In FIG. 1 a barge 24 is anchored over a pile 4 having its tip 12 embedded in sub-soil 100. The piles open mouth 30 is submerged. From the winch 25 on the barge descends a cable 26 through pile mouth 30 to a piston 10. The latter fits closely enough within the pile walls to at least partially and preferably substantially completely bar the entry of water into the space below the piston as it is raised, the need for or desireability of packing 11 being determined in part by the speed at which the piston is to be raised and lowered.

Operation of this embodiment simply involves repetitively and alternately raising the piston with winch 25 and dropping the piston, which may, if desired come to rest against a cushion block 13. Raising of the piston evacuates an enclosure defined by the pile tip and side walls. Quick release of the piston and rapid descent thereof through the pile accelerates a mass of water above the piston. This mass is suddenly decelerated by indirect contact through the rigid piston with the barrier provided by the cushion block 13 when the piston strikes the latter. This, in turn, generates the water hammer impulse which drives the pile.

To minimize drag and inertia forces which would retard the fall of piston 10, a clutch may be used to disengage the winch reel from its drive motor during the fall of the piston. For large piston loads a multi-sheaved block and tackle, mounted on the top of the hammer tube, may be employed. The main hook made in the form of a bull gear may be disengaged from the piston by rotation, pivoting around a hushed holding pin. The required mechanical power may be provided by a small electric or hydraulic motor-driven pinion. A small rope which follows the piston down its stroke may act as a guide for reengaging the lifting hook. Other quick make-break configurations are the commercially available wireline overshot latching clips for removing down-hole core barrels from diamond bits left inside petroleum walls.

A guiding long rack and motor-driven pinion means may also be used to raise the piston. A high pressureangle stubby gear tooth profile facilitates the easy disengagement of the pinion from the rack by a quickacting cam or hydraulic piston means. The rack can ride down with the piston and the pinion assembly remain fixed at the top of the hammer tube.

Similarly, another piston-raising means may employ a split nut fastened to hydraulicallyor cam-actuated chuck jaws to engage and quickly disengage a long, threaded screw fastened to the piston. The nut is rotated by a motor driven pinion meshing with a bull gear made integral with the chuck, all mounted in top of the water hammer tube.

Another method would use a hydraulically-actuated cylinder to lift the piston. A hydraulic chuck, on the end of the cylinder rod, latches and disengages the piston.

For shorter and faster piston hammer strokes a tubemounted electric or hydraulic motor-driven cam is used to provide a relatively slow lift and free drop to the piston.

Instead of packing or piston rings, a rolling diaphragm type seal may be used to keep the water out of the interior of the water hammer tube. A suitablyshaped fillet at the bottom of the stroke supports the elastomeric-impregnated fabric against the highamplitude water hammer pressure pulse.

In FIG. 2 is shown a closed tip pile 40 partially embedded in soil and completelysubmerged in the water. In the pile is temporarily mounted a driving module 41 comprising a base 42, central column 43, free piston 44 and associated parts. Steam supply and controls for the unit may be provided on a barge (not shown) anchored above the pile on the water surface. The base 42 is cylindrical, slightly less in diameter than the inside of pile 40, and includes any suitable means for releasably gripping the inside of the pile and maintaining the position of the base during driving. This may be accomplished, for instance, by inflatable gland 46, in peripheral groove 45, which presses outwardly against the pile. Gripping teeth may be provided on the outer surface of the gland. Various other mechanical and hydraulic coupling arrangements may also be employed.

The bottom central column 43 is secured in the center of the base. Its top may be secured in any desired manner, such as by spiders, leaving the top of the pile open. The column may also be supported by a plate having a gas tight seal 57, for example when it is desired to provide an over-pressure by pressuring the space above the water. The column is normally hollow, and is therefore a convenient channel for supply and control cables extending between the top and the base of the module. It also guides the piston 44.

Piston 44 is mounted for vertical or axial reciprocation on column 43 between base 42 and stop 50 secured near the upper end of the column. Both the piston and base are reinforced to withstand the mechanical shock associated with water hammer pressures. The piston has a central aperture 47 of slightly greater diameter than the outside of the column 43, and has an outer diameter slightly less than that of the inner diameter of the pile. Suitable seals may be provided if desired in the clearances between the piston on the one hand and the pile and column on the other. However, when operating with a small pressure differential across the piston, e.g., one atmosphere or less, leakage of water and steam past the piston will be minimal. Thus, it is possible to fabricate the apparatus in such a way as to provide a close but essentially drag free relationship between the piston and the other parts. Also, making the piston neutrally buoyant relative to water may reduce the pressure differential and discourage leakage.

Connected to steam supply hose 32 and control valve 33 is a steam conduit 48 which runs downwardly through column 43 into base 42 and has one or more outlets 49 beneath the piston. By suitable manual or automatic means the control valve 33 is opened to emit a burst of steam from the outlets 49 at a pressure greater than the ambient water pressure above the piston. In the preferred mode of operation, sufficient steam quantity, pressure and superheat are used so that the energy of the steam is expended before the piston reaches its upper limit of travel. When the momentum and further movement of the piston (away from the barrier, e.g. base 42) supercools the steam and condenses it, the interior of the pile is evacuated.

The ambient hydrostatic pressure in the water above the piston then accelerates the piston and the mass of water above it along the pile axis toward the base 42. The mass of water substantially fills the available crosssection of the pile and moves downwardly with the piston, both of which have velocity substantially independent of the pile. When the water is decelerated, it is at substantially its theoretical bulk modulus. The water is decelerated by indirect contact with base 42 through piston 44 and possibly some residual water. Condensation of the steam beneath the piston leaves the space between it substantially free of vapors and gases, but some condensed water may remain. Entrainedor entrapped gases such as air, if present, would introduce sufficient compliance into the system to very substantially reduce the mechanical impulse which results when the piston and water mass are suddenly decelerated against the barrier.

In order to keep the evacuatable enclosure free of steam condensate, and possibly of cooling water where such is used, the base 42 is fitted with a pump 51 which withdraws through a line 53 which passes up through central column 73 to a discharge point 54 above the upward limit of the pistons travel. Cooling water may be required when because of insufficient momentum in the piston, or for other reasons, there is not sufficient auto-cooling of the steam. In such case, cool water may be sprayed into the space beneath the piston by any suitable means (not shown) when it is near the top of its upstroke. If desired, part of the effluent from pump Sll may be routed to inflatable gland 46 via a line 56 in order to keep or help keep the gland inflated.

The bi-directional driver in FIG. 3 may be mounted for vertical sliding movement up and down a vertical bar (not shown) the bottom of which is secured to a suction base which acts as a supporting stand. The suction base provides strong resistance to overturning where such cannot be contributed by the pile (or core tube) itself. For example, a pressure differential of one atmosphere corresponds to over one ton of force per square foot. A relatively small, low pressure, motordriven water pump combination 89 maintains within the vacuum chamber 88, formed by the base plate 87 above and the perforated screen 93 below, a few psi of suction with respect to ambient water pressure. An interconnecting suction ductwork supports the screen. A peripheral elastomeric skirt 92 around the base reduces the external water leakage thereunder by lengthening the resistance of the water flow path. The openings in the strainer 93 reduce the particulate size to be handled by the large clearance pump 89. its discharge washes away the mud cleaned off the core tube 67 by the scraper rings 99. it is desirable that no base projections stick into the ground so that the base can be selfcentering around the core-tube to reduce driving frietion. To release, the base is flooded by either pump 6 or by reversing the pump 89.

The piston serves two primary purposes. First it acts as a hydrostatic pressure seal by means of the piston rings 91 bearing against both outer cylinder 4 and concentric inner cylinder 84, plus the face seals 28. Also, it preserves a coherent influent wave front, especially in the downward direction. A secondary driving function is applied by the kinetic energy in the pistons moving mass. Consequently the piston dimensions and its material density can be tailored to the peculiarities of particular driving and retrieval conditions. One cycle of pumping and valving operations will serve to illustrate the working process. Assume the upper piston ilt U has just completed a down-driving stroke and is resting on top of the lower piston L which is in the position as shown in FIG. 3. The lower piston 80L is hydromechanically clamped to remain stationary throughout the operating cycle by virtue of a high pressure fluid (HP) mechanically deflecting the thin walls of the elongated chamber 96L to seize against the piston sides.

This piston-holding function also may be provided by other clamping means 33, UL, UR, and L described later. The valve U connecting the upper end of the water hammer tube 4 to the pump 6 is used in the upward driving operation, but it remains shut during downward driving. The four-step sequence of valving operations is identified by the respective numerals 1-4 in the dotted circles drawn adjacent to the valves which are to be opened during that particular step, all others remaining closed; the respective arrows indicate flow direction. The first step involves raising the upper piston @tllU by pumping sea water 101 through the lower water tube vent 94L via value '7llLR, pump 6, valve 71 UL, and valve 95L. When the upper piston 80U reaches its upper stop, as shown in ETC. 3, it actuates a limit switch, not shown, which signal opens valve 96 introducing fluid under high pressure HP into cavity fioU to hydraulically clamp the upper piston 80U. Si-

multaneously in this second step, the pumping influx valves 71 UL and 71 LR are closed, and the efflux valves 71 UL and ul LL are opened. Hydraulic accumulators 85U and 85L, respectively, reduce the switching pressure transients. The amount of water tube evacuation (third step) determines the intensity of the water hammer blow. When the desired water level is reached, as monitored by a differential pressure cell, or level switch, the hydraulic pressure on clamp 86U is released through valve 97. Under the ambient hydrostatic head the freed upper piston 80U accelerates downward followed by the water column entering through upper ports 98U. The mechanical impact with the lower piston 80L, cushioned by the water level remaining in the tube 4, is transmitted to the core tube 67 by way of the mechanical clamps 83 series. The split, serrated jaws 99, which concentrically secure the core tube, are

made of compatible material with regard to hardness and friction. In the lower clamp 83L the clamping force is generated by a high hydraulic pressure HP in the cavity 103 behind the flexible elastomeric seal 102 bonding the jaws to the clamp walls. Scraping rings 90 clean the core tube of any clipping mud that would interfere with the clamping operation. A spring-return hydraulic cylinder type of alternate clamp is illustrated by 83 UL while a mechanical chuck clamp, represented by 83 UR is another choice. The 71 series of valves in FIG. 3 can be replaced by the 4-way valve 71 as shown in the schematic FIG. 3 A.

In the above-described embodiments, the water hammer tube has been entirely submerged in the water in which it is operating, the preferredmode of carrying out the invention. This makes use of the hydrostatic head available in the water to power the driving impulse. Also, the submerged-operation feature of the invention offers the possibility of easier handling during storm conditions. However, in other cases, especially shallow water applications, the water hammer tube may be at least partially above the surface of the water. Whether the evacuatable enclosure is defined by a tube separate from the pile, or by the pile itself, the water for generating the water hammer pulses maybe provided by an upward extension of the pile or the tube, which is filled with water, or by a reservoir located above the hammer tube as shown in FIG. 4.

In FIG. 4 is shown a pile 1 partially embedded in subsoil 100 and having its upper end 130 protruding above the waters surface. A driver 171 is releasably secured in the top of the pile by a coupler 140, the base 57 of which is coupled to the base 172 of the driver. To secure the driver within the pile, high pressure fluid is fed into the lower cylindrical cavity 153 of the pile'coupler 140 through port 154. This flow causes the cylinder frame 155 to move downward over the piston 156. The piston shaft is secured to the base 157 that is bolted to the bottom flange 148 of the water hammer tube .4. When the cylinder frame 155 moves downward it creates a toggle action in the multiplicity of links 158. The resultant mechanical advantage varies as the cotangent of the angle between the link and the radial normal. Consequently, hardtooth shoes 159 slide radially outward in T slot guides in the base 157 and bite into the pile walls. Simultaneously, the fluid in the upper cylindrical cavity' 160 is exhausted through port 161. A 4-way electrically controlled valve (not shown) can be used to control the influx and efflux of the pressurized fluid,'which may be hydraulic or air. To release the pile coupler, the influx and efflux ports on the piston base are interchanged by control valve action. The compression spring 162 in the upper cylinder retracts the entire mechanism when the air pressure is off.

Extending upwardly from base 172 is an upright, elongated water hammer tube 4. A reservoir 188 is supported by the tube 4 and connected thereto by flared walls 189 to promote smooth flow of water 190 between thetubes and reservoir. The reservoir may be pressurized if desired by forcing in gas or vapor through inlet 192. A central column 173 is suitably secured to the base 172 and extends upwardly and coaxially with the water hammer tube 4 and thence at least part way into reservoir 188 where it may be supported by a three-legged spider 191 secured to the'reservoir walls, only one leg of which is shown in the drawing.

Piston 174 is mounted for vertical or axial reciprocation on column 173 between base 172 and stop secured toward the upper end of the column. Both the piston and base 172 are reinforced to withstand the mechanical shock associated with water hammer pressures. The piston itself may of course contribute some driving momentum during operation, but normally, during driving, the mass of the piston is less, and usually substantially less than half, the mass of the fluid (water) which is above it or which enters the tube 4 during the down stroke.

The piston has a central aperture 177 of slightly greater diameter than the outside of column 173, and has an outer diameter slightly less than that of the inner diameter of the water hammer tube 4. Suitable seals may be provided-if desired in the clearances between the piston on the one hand and the pile and column on the other. However, when operating with a small pressure differential across the piston, e. g., one atmosphere or less, leakage of water and steam past the piston will be minimal. Thus, it is possible to fabricate the apparatus in a way which provides a close but essentially dragfree relationship between the piston and the other parts. Also, making the piston neutrally buoyant relative to water may reduce the pressure differential and discourage leakage.

Connected to a suitable steam supply (not shown) is a steam conduit 178 with control 'valve 123. Conduit 178 feeds passages in base 173 terminating in steam outlets 179. When the control valve 123 is opened to emit a burst of steam from the outlets 179 at a pressure greater than the ambient water pressure above piston 174, it will be forced upwardly in tube 4. When the piston retains sufficient upward momentum after control valve 123 closes, the resultant further expansion of the space beneath the piston can super-cool the steam and condense it, thus evacuating the space beneath the piston. Where, because of insufficient momentum or other reasons, there is not sufficient auto-cooling of the steam, cool water may be sprayed into the space beneath the piston by water conduits and spray nozzles (not shown) fitted into the central column and/or base, or into the side walls of tube 4.

In order to keep the evacuatable enclosure free of steam condensate, and possibly of cooling water where such is used, the base 172 may be fitted with a drain pipe 182 and valve 181. Valve 181, like steam valve 123, will normally be opened during the raising of piston 174 and closed on the down stroke.

In certain apparatus, e.g., that having a cam-actuated free piston it may be found desirable to adjust the actuation of the cam to maintain the hydraulic pulse repetition rate at an operational resonance of the system. This can be accomplished by placing sensors on the driver and/or pile and/or ground and automatically actuating the valve in response to signals from the sensors.

Although water is used as an example, the working fluid is not necessarily limited thereto. In a closed system, any liquid may be used.

Some contemporary offshore foundation designs call for loads up to 2000 tons from piles 200-600 ft. long, 3-8 ft. in diameter, weighing l'00200 tons, in up to 1000 ft. of water. Without supplementary techniques involving pre-drilling or jetting such piles are practically undrivable by the steam-air hammer even when spliced to extend to the surface.

From the foregoing, it may be seen that the invention provides many advantages. It makes feasible a large increase in driving capability. And this can be done using a smaller mass ratio (driving mass versus pile) than has heretofore been thought advisable in steam hammer operations. That is, large impulses can be generated using a driving mass which is less than one-fourth that of the pile. This, in turn, makes it possible to drive piles without the use of palliatives such as pilot holedrilling, water jetting and grouting into an oversized hole, which measures can reduce pile load-bearing capacity.

By varying enclosure length and diameter, the pressure-time characteristics of the water hammer impulse can be tailored over a wide range of values to match corresponding requirements of the pile and soil. Thus, driving impedance can be better matched to that of the earth than when operating with for instance a steel hammer.

Under the longer driving pulse generated by a water ram or spear with a length to diameter ratio of 10 or more, piles move more nearly as a unit, e.g., their driving action is more like that of a nail, rather than a worm, in which one part moves ahead while other parts are held back. Thus, a greater fraction of the driving energy is usefully expended in overcoming displacement skin friction, to advance the pile, rather than being tied up in the rubberlike ground quake. With the long pulses which may be provided with water hammer if desired, unwanted standing wave conditions in the pile can be prevented more effectively. The invention renders unnecessary the use of a cushion block, as sometimes required with a steel hammer, thereby eliminating the inelastic collision energy loss associated therewith.

Certain important advantages are associated with the convenient manner in which the invention may be applied under water. With the driver submerged, it may be handled with greater safety and ease during storm conditions. Coupling of the driver to the pile at a point below its top and helps to reduce losses of driving energy attributable to the mechanical compliance of the pile. Submerged operation provides inherent capacity for generating larger pulses as submergence increases, and particularly at depths greater than 200 feet where hydrostatic back pressure aggravates the venting prob-' lem of the air operated hammer, where thermal line losses preclude the steam driven hammer and where conventional vibratory driving requires such a relatively large back-mass for preload and such low frequencies that reaction forces necessary for driving become ineffective without excessively large excursions.

Handling is facilitated because the driving mass can be drained from the apparatus when it is being transported and lifted above the surface.

In view of the foregoing, it is apparent that the present invention is a broad one, and that many changes may be made in the foregoing embodiments without departing from the spirit of the invention.

What is claimed is:

l. A method of driving a pile having its tip embedded in the ground, an evacuatable enclosure with side walls and a lower barn'er being effectively coupled with the tip of the pile for transmitting driving forces exerted upon said barrier to said tip, said method comprising at least partially filling said enclosure with water, providing a hydrostatic head in said water bearing upon said barrier; providing piston means in said enclosure moveable longitudinally of said enclosure; evacuating a portion of said enclosure beneath said-piston means and pushing on said water with said piston means for displacing water along the longitudinal axis of the enclosure away from the barrier; releasing said piston and accelerating a mass of water following said piston along the axis of the enclosure towards said barrier under the influence of the hydrostatic head, the water and piston moving substantially independent of said pile; suddenly decelerating said piston and mass of water against said barrier, thereby converting hydraulic kinetic energy to a water hammer driving pulse for driving said pile into the ground; and repetitively evacuating, accelerating, decelerating, and driving as aforesaid.

2. A method in accordance with claim 1 wherein the tip of said pile is embedded in the sub-soil of a body of water.

3. A method in accordance with claim 2 wherein said evacuatable chamber is beneath the surface of said water.

4. A method in accordance with claim 2 wherein driving force is transmitted from said barrier to said pile through a coupling which is located below the top of said pile.

5. A method in accordance with. claim 4 wherein the driving force is transmitted through said coupling, said coupling being at a point closer to the sub-soil than to the top of said pile.

6. A method of driving a pile having its tip embedded in the ground, an evacuatable enclosure with side walls and a lower barrier being effectively coupled with the tip of the pile for transmitting driving forces exerted upon said barrier to said tip, said method comprising at least partially filling said enclosure with liquid, providing a hydrostatic head in said liquid bearing upon said barrier; providing piston means in said enclosure moveable longitudinally of said enclosure; evacuating a portion of said enclosure beneath said piston means and pushing on said liquid with said piston means for displacing liquid along the longitudinal axis of the enclosure away from the barrier; releasing said piston and accelerating a mass of liquid following said piston along the axis of the enclosure towards said barrier under the influence of the hydrostatic head, the liquid and piston moving substantially independent of said pile; suddenly decelerating said piston and mass of liquid against said barrier, thereby converting hydraulic kinetic energy to a liquid hammer driving pulse for driving said pile into the ground; and repetitively evacuating, accelerating,

decelerating, and driving as aforesaid. 

1. A method of driving a pile having its tip embedded in the ground, an evacuatable enclosure with side walls and a lower barrier being effectively coupled with the tip of the pile for transmitting driving forces exerted upon said barrier to said tip, said method comprising at least partially filling said enclosure with water, providing a hydrostatic head in said water bearing upon said barrier; providing piston means in said enclosure moveable longitudinally of said enclosure; evacuating a portion of said enclosure beneath said piston means and pushing on said water with said piston means for displacing water along the longitudinal axis of the enclosure away from the barrier; releasing said piston and accelerating a mass of water following said piston along the axis of the enclosure towards said barrier under the influence of the hydrostatic head, the water and piston moving substantially independent of said pile; suddenly decelerating said piston and mass of water against said barrier, thereby converting hydraulic kinetic energy to a water hammer driving pulse for driving said pile into the ground; and repetitively evacuating, accelerating, decelerating, and driving as aforesaid.
 2. A method in accordance with claim 1 wherein the tip of said pile is embedded in the sub-soil of a body of water.
 3. A method in accordance with claim 2 wherein said evacuatable chamber is beneath the surface of said water.
 4. A method in accordance with claim 2 wherein driving force is transmitted from said barrier to said pile through a coupling which is located below the top of said pile.
 5. A method in accordance with claim 4 wherein the driving force is transmitted through said coupling, said coupling being at a point closer to the sub-soil than to the top of said pile.
 6. A method of driving a pile having its tip embedded in the ground, an evacuatable enclosure with side walls and a lower barrier being effectively coupled with the tip of the pile for transmitting driving forces exerted upon said barrier to said tip, said method comprising at least partially filling said enclosure with liquid, providing a hydrostatic head in said liquid bearing upon said barrier; providing piston means in said enclosure moveable longitudinally of said enclosure; evacuating a portion of said enclosure beneath said piston means and pushing on said liquid with said piston means for displacing liquid along the longitudinal axis of the enclosure away from the barrier; releasing said piston and accelerating a mass of liquid following said piston along the axis of the enclosure towards said barrier under the influence of the hydrostatic head, the liquid and piston moving substantially independent of said pile; suddenly decelerating said piston and mass of liquid against said barrier, thereby converting hydraulic kinetic energy to a liquid hammer driving pulse for driving said pile into the ground; and repetitively evacuating, accelerating, decelerating, and driving as aforesaid. 